Abstract

The Jurassic to middle Eocene porphyry copper deposits and prospects exposed on the Pacific slopes of the central Andean Cordillera Occidental of southern Perú between latitudes 16°30' and 18° S record a protracted, ca. 30-m.y. history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment, itself a response to successive regional tectonic events, including the late Eocene Incaic, the late Oligocene to earliest Miocene Aymará, and the middle to late Miocene Quechuan events. Weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern Perú at 25.5 Ma following a 27-m.y. amagmatic interval, and supergene processes were variously interrupted or terminated by ignimbrite blanketing, although in several locations supergene profiles were preserved by such cover.

The landform chronology for the area surrounding the Cuajone, Quellaveco, and Toquepala deposits (ca. 17° S) is revised and extended northwestward through field mapping to the Cerro Verde-Santa Rosa district (ca. 16° 30' S). The 40Ar-39Ar incremental-heating dates of supergene alunite group minerals from the Angostura (38.1 and 38.8 Ma) and Posco (38.8 Ma) prospects and the Cerro Verde deposit (36.1–38.8 Ma) demonstrate that supergene processes were underway in the late Eocene beneath a subplanar topography resulting from uplift and erosion during the Incaic orogeny, now represented by a regional unconformity in the Cenozoic volcanic-sedimentary rock succession. Broadly contemporaneous supergene processes were probably active in the Cuajone-Quellaveco-Toquepala district. Slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid-Oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the Incaic supergene profiles. Aymará uplift subsequently led to the incision of the 23.8 to 24 Ma Altos de Camilaca and the 18.8 to 19.1 Ma Pampa Lagunas pediplains and their regional correlatives. The ensuing water-table lowering was associated with intense leaching and sulfide enrichment from the late Oligocene (24.4–28 Ma natroalunite at Cerro Verde, 26–27 Ma natroalunite at Santa Rosa, and 28.6 Ma jarosite at La Llave) to the early Miocene (23 Ma alunite and 21 Ma natroalunite at Cerro Verde, and 19.2 Ma jarosite at La Llave) and was plausibly responsible for much of the upgrading of the Cuajone and Toquepala deposits and thr Quellaveco prospect, which are intersected by both the Altos de Camilaca pediplain and erosional features representing upslope extensions of the Pampa Lagunas pediplain. The younger supergene profiles were widely superimposed on the remnants of those generated during the Incaic orogeny. Middle Miocene (<14.2 Ma biotite age) Chuntacala Formation flows protected the Cuajone supergene profile from destruction by erosion, but at 13.0 Ma interrupted supergene processes at Quellaveco. Revision of volcano-stratigraphic relationships in the latter area reveals that subsequent erosion of the Chuntacala Formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10.1 Ma ash-flow tuff of the Asana Formation. Elsewhere, supergene activity persisted at the Cachuyito prospect through 11.4 Ma, and minor jarosite development occurred at least until 4.9 Ma both there and at Cerro Verde during and following the Multiple Pediment landform stage (ca. 7.9–15.0 Ma).

The occurrence of relics of late Eocene alunite group minerals within considerably younger late Oligocene to late Miocene supergene alteration profiles suggests that the overall physiographic configuration of the Pacific piedmont of southern Perú remained remarkably consistent from the late Eocene to the middle Miocene. Moreover, the new age data confirm that, as in northern Chile, semiarid climatic conditions prevailed along much of the plate boundary from the mid-Eocene until the late Miocene or early Pliocene onset of hyperaridity.

The local geomorphologic and volcanic conditions in southern Perú, however, conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late Eocene to early Oligocene porphyry copper belt of northern Chile, which underwent supergene upgrading over relatively brief periods.

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